This invention relates to a novel method of processing of electrodes of the electron-gun mount assembly of a CRT (cathode-ray tube) to suppress afterglow during the subsequent operation of the CRT.
A CRT comprises an evacuated envelope which includes a neck, a funnel and a faceplate opposite the neck. A viewing screen is supported on the inner surface of the faceplate. A conductive coating supported on the inside surface of the funnel constitutes one plate of a filter capacitor and is the anode of the CRT. A conductive coating on the outside of the funnel constitutes the other plate of the filter capacitor. An electron-gun mount assembly, including one or more electron guns supported from a glass stem, is sealed into the neck of the envelope. Each electron gun includes a cathode and a plurality of electrodes including a final high-voltage electrode and a focus electrode spaced from the high-voltage electrode. After the mount assembly is sealed into the neck, the CRT is baked at about 300.degree. to 450.degree. C. and is simultaneously exhausted of gases to a relatively low pressure below 10.sup.-4 torr. Then, the CRT is tipped off; that is, the CRT is sealed. After tipping off, the mount assembly is subjected to electrical processing so that the electron guns become operative, spurious electron emission therefrom is reduced and their operation is stabilized.
A completed CRT, installed in a chassis and operated in a normal manner, may continue to emit light from the viewing screen after the normal operating voltages are removed from the mount assembly. This effect, which may linger for minutes or hours, is referred to as "afterglow" and is attributed to the coincidence of two factors. First, a large residual electrostatic charge remains on the filter capacitor (which is integral with the CRT) after the operating voltages are removed, and therefore a residual high voltage remains on the anode of the CRT and the high-voltage electrodes of the mount assembly with respect to the other electrodes of the mount assembly. Second, there are sites on the electrodes of the electron gun from which electrons can be emitted when they are under the influence of the electric field produced by the residual charge on the filter capacitor. Emitted electrons under the influence of the electric field are directed toward, and impinged upon, the viewing screen producing the afterglow.
It has been suggested previously that dipping the electrodes of the mount assembly in liquid air or a similar volatile liquid will induce localized turbulance in the liquid and thereby remove particles from the electrodes. It has also been suggested that dipping the electrodes of the mount assembly in an acid etching solution, which may contain hydrogen peroxide, for a sufficient time will remove oxidized layers that may have formed on the surfaces of the electrodes. These prior procedures are said to reduce arcing around the mount assembly during the operation of the CRT by reducing field emission. In both cases, material is removed from the surfaces of the electrodes.